There are three types of bits which are generally used to drill through subterranean formations, including percussion bits (also called impact bits), rolling cone bits, including tri-cone bits, and rotary drag bits or fixed cutter rotary bits (including core bits so configured). Rotary drag bits conventionally employ diamond or other superabrasive cutting elements or “cutters,” with the use of polycrystalline diamond compact (PDC) cutters being most prevalent.
In addition to conventional, concentric rotary drag and bits, there are other apparatus employed downhole and generically termed “tools” herein, which may be employed to cut or enlarge a borehole or which may employ superabrasive cutters, inserts or plugs on the surface thereof as cutters or wear-prevention elements. Such tools include, without limitation, bicenter bits, eccentric bits, expandable reamers, and reamer wings.
It has been known in the art for many years that PDC cutters perform well on drag bits and other rotary tools. A PDC cutter typically has a diamond layer or table formed under high temperature and pressure conditions to a cemented carbide substrate (such as cemented tungsten carbide) containing a metal binder or catalyst such as cobalt. The substrate may be brazed or otherwise joined to an attachment member such as a stud or to a cylindrical backing element to enhance its affixation to the bit face. The cutting element may be mounted to a drill bit either by press-fitting or otherwise locking the stud into a receptacle on a steel-body drag bit, or by brazing the cutter substrate (with or without cylindrical backing element) directly into a preformed pocket, socket or other receptacle on the face of a bit body, as on a matrix-type bit formed of WC particles cast in a solidified, usually copper-based, binder as known in the art.
A PDC is normally fabricated by placing a disk-shaped, cemented carbide substrate into a container or cartridge with a layer of diamond crystals or grains loaded into the cartridge adjacent one face of the substrate. A number of such cartridges are typically loaded into an ultra-high pressure press. The substrates and adjacent diamond crystal layers are then compressed under ultra-high temperature and pressure conditions. The ultra-high pressure and temperature conditions cause the metal binder from the substrate body to become liquid and sweep from the region behind the substrate face next to the diamond layer through the diamond grains and act as a reactive liquid phase to promote a sintering of the diamond grains to form the polycrystalline diamond structure As a result, the diamond grains become mutually bonded to form a diamond table over the substrate face, which diamond table is also bonded to the substrate face. The metal binder may remain in the diamond layer within the pores existing between the diamond grains or all or a portion of the metal binder may be removed, as well known in the art. The binder may be removed by acid leaching or an electrolytic leaching process. For more background information concerning processes used to form polycrystalline diamond cutters, the reader is directed to U.S. Pat. No. 3,745,623, issued on Jul. 17, 1973, in the name of Wentorf, Jr. et al., the disclosure of which patent is incorporated by reference herein.
An embodiment of a conventional rotary drag bit is shown in FIG. 1. The drag bit of FIG. 1 is designed to be turned in a clockwise direction (looking downward at a bit being used in a hole, or counterclockwise if looking at the bit from its leading end, or face as shown in FIG. 1) about its longitudinal axis. The majority of current drag bit designs employ diamond cutters comprising PDC diamond tables formed on a substrate, typically of cemented tungsten carbide (WC) State-of-the-art drag bits may achieve a rate of penetration (ROP) under appropriate weight on bit (WOB) and applied torque, ranging from about one to in excess of one thousand feet per hour. A disadvantage of state-of-the-art PDC drag bits is that they may prematurely wear due to impact failure of the PDC cutters, as such cutters may be damaged very quickly if used in highly stressed or tougher formations composed of limestones, dolomites, anhydrites, cemented sandstones, interbedded formations, also known as transition zones, such as shale with sequences of sandstone, limestone and dolomites, or formations containing hard “stringers.” As noted above, there are additional categories of tools employed in boreholes, which tools employ superabrasive cutting elements for cutting, and which suffer the same deficiencies in the drilling the enumerated formations. In many such formations, other types of cutting structures have been employed in drag bits, including small natural diamonds, small so-called “thermally stable” PDC cutters, and diamond grit-impregnated metal carbide matrix-type cutting structures of various configurations. However, such drag bits provide a much-inferior ROP to PDC cutter-equipped bits and so incur substantial additional drilling cost in terms of rig and drilling crew time on site.
Conventional PDC cutters experience durability problems in high load applications. They have an undesirable tendency to crack (including microcracking), chip, spall, and break when exposed to hard, tough or highly stressed geologic structures so that the cutters consequently sustain high loads and impact forces. They are similarly weak when placed under high loads from a variety of angles. The durability problems of conventional PDCs are worsened by the dynamic nature of both normal and torsional loading during the drilling process, wherein the bit face moves into and out of contact with the uncut formation material forming the bottom of the wellbore, the loading being further aggravated in some bit designs and in some formations by so-called bit “whirl.”
The diamond table/substrate interface of conventional PDCs is subject to high residual stresses arising from formation of the cutting element, as during cooling, the differing coefficients of thermal expansion of the diamond and substrate material result in thermally induced stresses. In addition, finite element analysis (FEA) has demonstrated that high tensile stresses exist in a localized region in the outer cylindrical substrate surface and internally in the substrate. Both of these phenomena are deleterious to the life of the cutting element during drilling operations as the stresses, when augmented by stresses attributable to the loading of the cutting element by the formation, may cause spalling, fracture or even delamination of the diamond table from the substrate.
Further, high tangential loading of the cutting edge of the cutting element results in bending stresses on the diamond table, which is relatively weak in tension and will thus fracture easily if not adequately supported against bending. The metal carbide substrate on which the diamond table is formed may be of inadequate stiffness to provide a desirable degree of such support.
The relatively rapid wear of diamond tables of conventional PDC cutters also results in rapid formation of a wear flat in the metal carbide substrate backing the cutting edge, the wear flat reducing the per-unit area loading in the vicinity of the cutting edge and requiring greater weight on bit (WOB) to maintain a given rate of penetration (ROP). The wear flat, due to the introduction of the substrate material as a contact surface with the formation, also increases drag or frictional contact between the cutter and the formation due to modification of the coefficient of friction. As one result, frictional heat generation is increased, elevating temperatures in the cutter and initiating damage to the PDC table in the form of heat checking while, at the same time, the presence of the wear flat reduces the opportunity for access by drilling fluid to the immediate rear of the cutting edge of the diamond table.
There have been many attempts in the art to enhance the durability of conventional PDC cutters by modification of cutting face geometry, specifically in the vicinity of the cutting edge which engages the formation being drilled. By way of example, the reader is directed to U.S. Pat. RE32,036 to Dennis (the '036 patent); U.S. Pat. No. 4,592,433 to Dennis (the '433 patent); and U.S. Pat. No. 5,120,327 to Dennis (the '327 patent). In FIG. 5A of the '036 patent, a cutter with a beveled peripheral edge is depicted, and briefly discussed at Col. 3, lines 51-54. In FIG. 4 of the '433 patent, a very minor beveling of the peripheral edge of the cutter substrate or blank having grooves of diamond therein is shown (see Col. 5, lines 1-2 of the patent for a brief discussion of the bevel). Similarly, in FIGS. 1-6 of the '327 patent, a minor peripheral bevel is shown (see Col. 5, lines 40-42 for a brief discussion of the bevel). Such bevels or chamfers were originally designed to protect the cutting edge of the PDC while a stud carrying the cutting element was pressed into a pocket in the bit face. However, it was subsequently recognized that the bevel or chamfer protected the cutting edge from load-induced stress concentrations by providing a small load-bearing area which lowers unit stress during the initial stages of drilling. The cutter loading may otherwise cause chipping or spalling of the diamond layer at an unchamfered cutting edge shortly after a cutter is put into service and before the cutter naturally abrades to a flat surface, or “wear flat,” at the cutting edge.
It is also known in the art to radius, rather than chamfer, a cutting edge of a PDC cutter, as disclosed in U.S. Pat. No. 5,016,718 to Tandberg. Such radiusing has been demonstrated to provide a load-bearing area similar to that of a small peripheral chamfer on the cutting face.
For other approaches to enhance cutter wear and durability characteristics, the reader is also referred to U.S. Pat. No. 5,437,343 to Cooley et al. (the '343 patent); and U.S. Pat. No. 5,460,233 to Meany et al. (the '233 patent), assigned to the assignee of the present invention. In FIGS. 3 and 5 of the '343 patent, it can be seen that multiple, adjacent chamfers are formed at the periphery of the diamond layer (see Col. 4, lines 31-68 and Cols. 5-6 in their entirety). In FIG. 2 of the '233 patent, it can be seen that the tungsten carbide substrate backing the superabrasive table is tapered at about 10-15° to its longitudinal axis to provide some additional support against catastrophic failure of the diamond layer (see Col. 5, lines 2-67 and Col. 6, lines 1-21 of the '233 patent). The disclosures of each of the '343 patent and the '233 patent are incorporated by reference herein. See also U.S. Pat. No. 5,443,565 to Strange for another disclosure of a multi-chamfered diamond table.
It is known that conventionally providing larger chamfers on cutters enhances durability, but at the same time reduces ROP and undesirably increases required WOB for a given ROP. The increased WOB translates to more energy applied to the drilling system, and specifically the drag bit which, in turn, stimulates cutter damage.
U.S. Pat. No. 5,706,906 to Jurewicz et al., assigned to the assignee of the present invention and the disclosure of which is incorporated by reference herein, describes PDC cutters of substantial depth or thickness, on the order of about 0.070 inch to 0.150 inch and having cutting faces with extremely large chamfers or so-called “rake lands” on the order of not less than about 0.050 inch, as measured radially along the surface of the rake land.
A PDC cutter as described in the '906 patent has demonstrated, for a given depth of cut and formation material being cut, a substantially enhanced useful life in comparison to prior art PDC cutters due to a greatly reduced tendency to catastrophically spall, chip, crack, and break. It has been found that the cutter in PDC form may tend to show some cracks after use, but the small cracks do not develop into a catastrophic failure of the diamond table as typically occurs in PDC cutters. This capability, if fully realized, would be particularly useful in a cutter installed on a drag bit to be used on hard rock formations and softer formations with hard rock stringers therein (mixed interbedded formations).
While such PDC cutters, with their large rake lands, have shown some promise in initial field testing, conclusively proving the durability of the design when compared to other cutters of similar diamond table thickness but without the large rake land, these PDC cutters also demonstrated some disadvantageous characteristics which impaired their usefulness in real-world drilling situations. Specifically, drill bits equipped with these PDC cutters demonstrated a disconcerting tendency, apparently due to the extraordinarily great cutting forces generated by contact of these cutters with a formation being drilled, to overload drilling motors, other bottomhole assembly (BHA) components such as subs and housings, as well as tubular components of the drill string above the BHA.
Further, bits equipped with these PDC cutters often drilled significantly slower, that is to say, their rate of penetration (ROP) of the formation was far less than, the ROP of bits equipped with conventional PDC cutters, and also exhibited difficulty in drilling through hard formations for which they would be otherwise ideally suited. It appears that the exterior configuration of these thick diamond table cutters, although contributing to the robust nature of the cutters, may be less than ideal for many drilling situations due to the variable geometry of the arcuate rake land as it contacts the formation and attendant lack of “aggressiveness” in contacting and cutting the formation. It is conceivable, as demonstrated in the cutting of metal with similarly shaped structures, that in plastic formations these PDC cutter may simply deform the material of the formation face engaged by the cutter, forming a plastic “prow” of rock ahead and flanking the cutter, instead of shearing the formation material as intended.
Therefore, despite the favorable characteristics exhibited by these PDC cutters, their utility in efficiently cutting the difficult formations for which its demonstrated durability is ideally suited remains, as a practical matter, unrealized over a broad range of formations and drilling conditions.
U.S. Pat. No. 5,881,830 to Cooley, assigned to the assignee of the present invention and the disclosure of which is incorporated by reference herein, describes PDC cutters having cutting faces with a first portion transverse to a longitudinal axis of the cutter and a second portion comprising a planar engagement surface or buttress plane oriented at a small, acute angle to the first portion and having a cutting edge along at least a portion of its periphery. These PDC cutters are described as durable, fairly aggressive and providing a more consistent performance over the life of the cutter than the PDC cutters described in the '906 patent, but their large chamfers result in an unacceptable reduction in aggressivity in cutting, leading to a reduced ROP.
In addition, U.S. Pat. No. 6,935,444 to Lund et al., assigned to the assignee of the present invention and the disclosure of which is incorporated by reference herein, discloses the use of multiple, adjacent chamfers having an arcuate surface located therebetween along a cutting edge of a PDC cutter. Such a geometry has been demonstrated to inhibit initial chipping of a PDC cutter along the cutting edge, prolonging the life thereof.
However, and as noted with regard to the PDC cutter designs discussed above, there remains a need for a robust superabrasive cutter which will withstand cutting stresses in the difficult formations referenced above and exhibit reduced wear tendencies while drilling effectively with, and without damage to, conventional, state-of-the-art bottomhole assemblies and drill strings, while providing commercially viable, consistent ROP.
During laboratory testing, it has been observed that conventional, 45° chamfer angle cutters with conventional chamfer depths on the order of, for example, 0.016 inch, commonly experience premature cutter damage and failure when the wear flat extends inwardly of the inner boundary of the chamfer. Specifically, an increased incidence of spalling and chipping of the PDC table has been observed. This is a particular problem in the aforementioned highly stressed or tougher formations, interbedded formations and formations containing hard stringers.
Several factors are believed to contribute to these types of cutter failure. First, during a drilling operation, downward force is applied to the competent formation under WOB as a result of chamfer and cutter backrake angle, maintaining the PDC table in compression and adding to cutter integrity. However, when the inner edge or boundary of the chamfer is worn away, the chamfer component of the compressive forces is diminished, with a consequent potential for high tensile shear forces to be present at the cutting face, resulting in the aforementioned spalling and chipping. Further, when the inner edge or boundary of the chamfer is worn away, a sharp edge or corner at the cutting face is presented to the formation, similar to that presented by an unchamfered cutter. Any vertical (parallel to the plane of the cutting face) forces acting on this sharp edge will translate as vertical tensile shear across the cutting face, resulting in a spatted cutter.
In addition, heat checking in the PDC table, due to the initiation of a large, relatively wide wear flat is particularly significant toward the rear of the wear flat and may result in significant breakage of the PDC table at the back and sides thereof.